PyCQA / pylint / 728

92%

main: 95%

Pull #1115

travis-ci

Add a Python 3 warning for implementing `__eq__` without also implementing `__hash__`

I've been exploring transitioning our codebase to Python 3, and I've been using the immensely helpful `-3` flag
when running tests as described here:

https://docs.python.org/3/howto/pyporting.html#prevent-compatibility-regressions

Unfortunately, one of the most common issues I'm finding is around classes implementing `__eq__` but not
`__hash__`.  The behavior around this changed in Python 3.  See below:

```python
class JustEq(object):
   def __init__(self, x):
     self.x = x
   def __eq__(self, other):
     return self.x == other.x

class Neither(object):
  def __init__(self, x):
    self.x = x

class HashAndEq(object):
   def __init__(self, x):
     self.x = x
   def __eq__(self, other):
     return self.x == other.x
   def __hash__(self):
     return hash(self.x)

{Neither(1), Neither(2)}  # OK
{HashAndEq(1), HashAndEq(2)}  # OK
{JustEq(1), JustEq(2)}  # Works in Python 2, throws in Python 3

as_set = {JustEq(1), JustEq(2)}
print(JustEq(1) in as_set)  # prints False
print(JustEq(1) in list(as_set))  # prints True
```

In general this is bad practice for the reasons above, so I'd actually love to havet this warn
both as a general warning and as a Python 3 warning, but I don't see a clean way to have the same
rule apply in multiple contexts.

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